1. Field of the Invention
This invention relates to an X-ray CT apparatus for reconstructing a tomographic image by the use of projection data.
2. Related Background Art
In an X-ray CT (computer tomography) apparatus, there is practiced half scan reconstruction which reconstructs an image by the use of projection data of 180 degrees+fan angle without the use of projection data corresponding to one full rotation. Further, it is also known that when use is made of projection data of 180 degrees+fan angle to 360 degrees, reconstruction can be done by a similar method as disclosed in Japanese Patent Application Laid-Open No. 2001-299738.
Describing this by the use of sinogram, in a system construction by an X-ray detector 2 opposed to an X-ray source 1 shown in FIG. 9 of the accompanying drawings with a subject S interposed therebetween, a sinogram is an image formed by projection data arranged with the fan angle or channel direction of the X-ray detector 2 as the axis of abscissas and the projection angle direction of the X-ray source 1 as the axis of ordinates, as shown in FIG. 10 of the accompanying drawings. Herein, description is made with respect to an arcuate X-ray detector 2 and therefore, the axis of abscissas is the fan angle, but in the case of a linear X-ray detector 2, conversion from distance to angle is necessary.
In a case where a fan beam is used as the X-ray source 1, as shown in FIG. 11 of the accompanying drawings, an X-ray transmission route of a fan angle α coincides with an X-ray transmission route of which the fan angle is −α when a projection angle is rotated by 180 degrees +2α. This means that assuming that the transmission data value of a transmission route of a fan angle α and a projection angle β is g(α,β), the transmission data value g(−α,Π+β+2α) of a transmission route opposed thereto coincides therewith. That is, g(α,β)=g(−α,Π+β+2α). This is represented on the sinogram as shown in FIG. 12 of the accompanying drawings.
Thereby, it will be seen that when the projection angle is 0 degree to 180 degrees +2δ(δ being the one side maximum angle of the fan), a gray portion shown in FIG. 13 of the accompanying drawings is dually overlapping data. Thus, if this overlapping data portion is corrected by a weighting factor so as to become the same weight as a non-duplicate portion, it can be normally reconstructed.
Even in the case of a projection angle greater than 180 degrees+fan angle α, reconstruction can be accomplished by the use of a weighting factor as in half scan reconstruction.
The reconstruction used in the actual CT system is either full scan reconstruction using projection data fully 360 degrees or half scan reconstruction using projection data of 180 degrees+fan angle α. In the case of the half scan reconstruction, projection data used for the reconstruction is substantially a half and therefore, the quality of image is insufficient.
Also, in the case of full scan reconstruction, projection data used becomes double and correspondingly, the time required for image taking becomes double, and the risk of an inconvenience such as, for example, body movement or the fluctuation of the quality of X-ray occurring during scanning becomes high, and if such inconvenience occurs, a construction may occur to the projection data and in some cases, an artifact may occur or the deterioration of the quality of image may occur.
Reconstruction in a case when use is made of projection data of a 0 degree−180 degrees+fan angle α to 360 degrees is also known, but no consideration is given to what range of projection data should be used.